What is the ruling on ablution (Wudu') and prayer (Salat) in the event of excessive vaginal discharge? Is it necessary to change undergarments after its emission? And is the prayer valid if some discharge is emitted after ablution and before prayer?

Vaginal discharge is pure (tahir), and there is no obligation to perform istinja (ablution after defecation/urination) from it, nor to purify clothes or the body that it touches. However, it does nullify wudu (ablution) according to the majority of scholars. A woman who is afflicted with continuous discharge should perform wudu for each prayer after its time has entered, and she may pray the fard (obligatory) and nawafil (supererogatory) prayers with that wudu. It is not permissible for her to perform wudu before the time of prayer enters; if she does, her prayer is invalid according to the majority of scholars. If she is certain that the discharge is coming from the urinary opening, then it is impure (najis); otherwise, it is pure, because the default is that it originates from the birth canal.